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From the Cassini-Huygens Website - Huygens Mission to Titan:

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This page is copied directly from the Huygens website! It is to serve as a sort of mirror in case the Huygens page becomes old and unlisted.
 

This artist's conception shows Titan's surface with Saturn appearing dimly in the background through Titan's thick atmosphere of mostly nitrogen and methane. The Cassini spacecraft flies overhead with its high-gain antenna pointed at the Huygens probe as it nears the surface.

Titan's surface may hold lakes of liquid ethane and methane, sprinkled over a thin veneer of frozen methane and ammonia. Most of the brownish-orange color comes from more heavily processed hydrocarbons present in Titan's atmosphere and on its surface. Artistic license has been used to exaggerate the size of the orbiter, the sharpness of the icy features, the tilt of Saturn's rings, and the visibility of the planet through Titan's atmosphere. By Craig Attebery

The European Space Agency's Huygens probe ushered in 2005 with its landmark mission at Titan. After a seven-year journey bolted to the side of the Cassini Orbiter, Huygens was set free on Dec. 25, 2004. The Probe coasted for 21 days en route to Titan.

Probe Separation and Transit to Titan
Prior to the probe's separation from the orbiter, the triplicate "coast" timer, or Mission Timer Unit (MTU) was loaded with the precise time necessary to turn on the probe systems (about 4 hours before the initial encounter with Titan's atmosphere). Then the probe separated flawlessly from the orbiter. Cassini turned and imaged Huygens repeatedly as it set out on its 21-day coast to Titan, with no systems active except for its wake-up timer.

Huygens separated from Cassini at 30 centimeters (about 12 inches) per second and a spin rate of seven revolutions per minute to ensure stability during the coast and entry phase. This rate was confirmed by the Magnetometer instrument on the Cassini Orbiter. Five days following the release of the probe, Cassini performed a deflection maneuver. This placed the orbiter on the proper trajectory - missing Titan instead of impacting - to collect Huygens' data during the probe mission.

Titan's nitrogen-rich atmosphere extends 10 times further into space than Earth's atmosphere. This means the outer fringes of Titan's atmosphere reach almost 600 kilometers (almost 400 miles) into space. When the probe detected this region of Titan's atmosphere, the deceleration set off a sequence of events leading to its perfect parachute descent.

Huygens was equipped with six science instruments designed to study the content and dynamics of Titan's atmosphere and collect data and images on the surface.

Mission Timeline Facts
All times are given in Spacecraft Event Time (SCET), Universal Coordinated Time (UTC)
 
Probe Release:
December 25, 2004 02:00 UTC
Probe entry at Titan:
January 14, 2005 11:04 UTC
Speed at Entry:
6 kilometers per second
Impact Speed:
5 meters per second
Mission Duration:
2 hr, 27 min, 13 sec descent, plus
1 hr, 12 min, 9 sec surface
Altitude of Cassini during the Huygens Mission:
60,000 kilometers
Data rate to Cassini Orbiter:
8 kilobits per second
Total Battery Capacity of Probe =
1800 Watt-hours

 

Descent Through Titan's Atmosphere
Huygens made a parachute-assisted descent through Titan's atmosphere, collecting data as the parachutes slowed the probe from super sonic speeds. Five batteries onboard the probe were originally sized for a Huygens mission duration of 153 minutes, corresponding to a maximum descent time of 2.5 hours plus a half hour or more on Titan's surface. In fact, they lasted much longer than that. These batteries were capable of generating a total of 1800 Watt-hours of electrical power.

The probe's radio link was activated early in the descent phase, during time the orbiter was flying overhead, "listening" for the probe. Not long after the end of this four-hour communications window, Cassini's high-gain antenna (HGA) turned away from Titan and pointed toward Earth.
 
This artist's conception of the Cassini orbiter shows the Huygens probe separating to enter Titan's atmosphere. After separation, the probe drifts for about three weeks until reaching its destination, Titan. Equipped with a variety of scientific sensors, the Huygens probe will spend 2-2.5 hours descending through Titan's dense, murky atmosphere of nitrogen and carbon-based molecules, beaming its findings to the distant Cassini orbiter overhead. The probe could continue to relay information for up to 30 minutes after it lands on Titan's frigid surface, after which the orbiter passes beneath the horizon as seen from the probe.

The peak heat-flux was expected in the altitude range below 350 kilometers (217 miles) down to 220 kilometers (137 miles), where Huygens rapidly decelerated from about 21,600 kilometers (13,424 miles) per hour to 1,440 kilometers (895 miles) per hour in less than two minutes.

At this speed, the parachute deployment sequence initiated, starting with a mortar pulling out a Pilot Parachute which, in turn, pulled away the aft cover and deployed the Main Parachute. After inflation of the 8.3 meter (27.2 foot) diameter main parachute, the front shield was released to fall from the Descent Module. Then, after a 30 second delay built into the sequence to ensure that the shield was sufficiently far away to avoid instrument contamination, the Gas Chromatograph Mass Spectrometer (GCMS) and Aerosol Collector and Pyrolyser (ACP) inlet ports opened and the Huygens Atmospheric Structure Instrument (HASI) boom deployed. The Descent Imager/Spectral Radiometer (DISR) cover was ejected two minutes later.

The main parachute was sized to pull the Descent Module safely out of the front shield. It was jettisoned after 15 minutes to avoid a protracted descent, and a smaller 3-meter (10-foot) diameter parachute was deployed. The descent lasted about two and a half hours.

During its descent, Huygens' camera returned more than 750 images, while the Probe's other five instruments sampled Titan's atmosphere to help determine its composition and structure. Huygens collected 2 hours, 27 minutes, 13 seconds of descent data, and 1 hour, 12 minutes, 9 seconds of surface data, which turned out to be far more surface data than was ever expected.

Every bit of data from Huygens was successfully relayed to the Cassini Orbiter passing overhead, with the exception of a redundant stream called "Chain A." Chain A's radio frequency was based on Huygens' Ultra Stable Oscillator, designed for the Doppler Wind Experiment. While this signal was not received aboard Cassini, it was received on Earth, thanks to Radio Scientists using Earth-based radio telescopes at Green Bank and Parkes. They were actually able to capture the tiny signal, now being called "Chain C," directly from Titan! Telemetry data from Huygens was stored onboard Cassini's Solid State recorders (SSR) for playback to Earth. Huygens is managed by the European Space Agency. Complete details on the mission objectives and science can be found on the ESA Huygens Site.

 

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